A human telomeric DNA includes a sequence in which a double-stranded DNA composed of 5′-TTAGGG-3′ (SEQ ID NO: 1) and 5′-CCCTAA-3′ (SEQ ID NO: 2, complementary strand of SEQ ID NO: 1) is repeated and a single-stranded DNA at its terminal end in which only 5′-TTAGGG-3′ (SEQ ID NO: 1) is repeated. The sequence in which 5′-TTAGGG-3′ (SEQ ID NO: 1) is repeated can form a quadruplex DNA structure called a G-quadruplex. The G-quadruplex is a structure constituted such that 4 guanines form a structure called a G-quartet, which in turn, stacks on one another through a π-π stacking interaction (FIG. 1). The G-quadruplex has been extensively investigated in recent years because it is considered to be associated with canceration and life of cells.
Genome-wide analyses that were recently performed with computers showed that a large number of sequences considered to be capable of forming a G-quadruplex existed in genome DNA in addition to telomeric DNA. Many of them exist in promoter regions of oncogenes including c-kit, c-myc, H-ras and K-ras genes. Therefore, these sequences considered to be capable of forming a G-quadruplex are also investigated. The fact described above suggests the possibility that the G-quadruplex plays an important role in the activity of cells.
Under the background described above, there is a need to provide a technique for conveniently analyzing whether or not a DNA considered to be capable of forming a G-quadruplex can really form a G-quadruplex. Particularly, the potassium ion concentration in the cell is about 100 to 150 mM, and therefore a technique capable of analyzing formation of a G-quadruplex under this potassium ion concentration condition is necessary. On that account, a compound that generates an especially strong fluorescence when reacting with a G-quadruplex as compared to a single-stranded DNA or a double-stranded DNA (hereinafter, referred to as a G-quadruplex probe) has been explored. In other words, the G-quadruplex probe must have a nature of generating little fluorescence when reacting with a single-stranded DNA or a double-stranded DNA, but generating a strong fluorescence when reacting with a G-quadruplex.
One of G-quadruplex probes that have been most extensively investigated in recent years is a benzothiazole derivative. The reason why the benzothiazole derivative is extensively investigated is that the benzothiazole derivative has a high water solubility and a very large variation in fluorescence intensity. For example, NPL 1 reports a G-quadruplex detection technique using Cyan 40 (chemical formula 1) and Cyan 2 (chemical formula 2). This report shows that when Cyan 2 is reacted with a G-quadruplex under such conditions that potassium ions are not present, a significantly strong fluorescence is generated as compared to a case where Cyan 2 is reacted with a double-stranded DNA. However, it is also shown that in the presence of 100 mM potassium ions, little fluorescence is detected even when Cyan 2 is reacted with a G-quadruplex. Therefore, Cyan 2 cannot be used for detection of a G-quadruplex in the presence of potassium ions. NPL 2 reports a G-quadruplex detection technique using thiazole orange (chemical formula 3) (hereinafter, referred to as TO). It is shown in this report that TO generates a strong fluorescence when it is reacted with a G-quadruplex in the presence of 100 mM potassium ions. However, it is also shown that when TO is reacted with a double-stranded DNA under the same conditions, TO generates a stronger fluorescence as compared to a case where it is reacted with a G-quadruplex. Therefore, the G-quadruplex cannot be specifically detected using TO in the presence of 100 mM potassium ions.

As described above, in recent years, a technique has been under development in which a G-quadruplex is specifically detected using a benzothiazole derivative as a G-quadruplex probe. However, a technique capable of specifically detecting a G-quadruplex under such conditions that potassium ions are present as in intracellular conditions has not been known.